Thermostat



Aug. 9, 1932. v. BUSH 1,870,235

TH-ERMOSTAT Filed Sept. 6, 1930 V/////////////////////////// //f47//////// WWW \\\k\\\\\\\\\\\\\ fi\\\.\\

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4 Illlllll Patented Aug. 9, 1932 UNITED STATES PATENT OFFICE VANNEVA'B BUSH, OF BELMONT/MASSACHUSETTS, ABSIGNOB 'I'O GENERAL PLATE OOH- PANY, OI ATTLEBOBO, MASSACHUSETTS, A CORPORATION OF MASSACHUSETTS THEBHOSTAT Application. fled September 8, 1980. Serial No. 480,078.

This invention relates to thermostats, and with regard to certain more specific features,

to a thermostat having a sheet of more than two metallic or like components, varying from three to an infinite number.

straints; the provision of a sheet of the class described which will wear longer under repeated straining; and the provision of a sheet of the class described from which an imroved snap-acting, thermostatic device may e made by using only a single piece of said sheet. Other objects will be in" part obvlous and in part pointed out hereinafter. Y a The invention accordingly comprises the elements and combinations of elements, features of construction, and arrangements of parts which will be exemplified in the structure hereinafter described, and the scope of the application of which will be indicated in the following claims.

In the accompanying drawing, in which are illustrated several of various possible embodiments of the invention,

Fig. 1 is a diagrammatic section'showing one form of the invention;

Fig. 2 is a view showing another form;

Fig- 3 is a cross section showing a thermostatic sheet fabricated into a switch;

' the composite-sheet, thermostatic art. These sheets are relatively thin (a few thousandths of an inch) and, in the form of the invention under discussion, comprise a number greater than two, four being shown by wav of example. The greater the number, the more improvement is shown, as will be seen heremafter.

Each sheet preferably has a thickness equal to that of its neighbor, although this is not a rigid requirement, and each has a thermal coeflicient of expansion which varies from that of its neighbor, suitable materials being chosen to accomplish the latter effect, such as steel, copper nickel and the like and their alloys such as invar, nickel, steel and the like. The discrete layers 3, 5, 7, 9 should have their 'coeflicient of expansion differ progressively that is, the sheets should be arranged in order of magnitude of these coeflicients.

From the above. it will be apparent that Fig. 1 is representative of a composite thermostatic sheet comprising a finite number of layers greater in number than two.

In Fig. 2 is illustrated a composite sheet 11 composed of an infinite number of infinitesimal gradations of material having a graded, progressive range of thermal coeflicients of expansion as the thickness of the sheet is traversed. This result may be accomplished for example by plating on a thin nickel cathode plate from a pair of nickel and copper anodes and adjusting the current at the start so that pure nickel is first plated and no copper.

Then the amount of copper plated is increased.

(at which time the amount of nickel plating may be decreased gradually, if desired) until at the opposite surface of the finished sheet there is had practically Monel. Or th.e baths used may be changed gradually with or without anode changes. After plating it is desirable to heat treat the resulting sheet and roll it inordertoobtainpropergraining. Thusthere is effected a series of infinitesimal difierences in metallic constituents as the thickness of the sheet is traversed, the thermal coefficient of expansion varying accordingly. Such a sheet 11 (Fig. 2) will be treated herein as comprising an infinite number of layersv of infinitesimal thicknesses having different thermal coefiicients of expansion of progressive order and also varying infinitesimally. It will be seen that the thermal coeflicient of expansion variation may be according toa straight line or curved line diagram when plotted against distance through the plate,

as was the case with the finite number of stri s.

e theory of operation of the material is as follows:

First will be considered ordlnary bimetal.

- We have, then, a thin strip of thermostatic and the stress diagram will show zero stress at the center line. The tension on the top layer, and the compression on the bottom layer, are equal.

Next consider the strip clamped fiat and raised to a higher temperature, the clamping being such that no end thrustis produced. The compression, uniform throughout the top material, is equal to the tension, uniform throughout the bottom material. Thus instead of zero stress at the center line, there will be a finite stress of some magnitude.

Now consider the free bimetallic strip heated to a given temperature. It will bend into a circular arc, and the stresses along the centenplane will be exactly as in the case of the strip clamped flat and raised in temperature, for bending does not alter these stresses.

The .final case to be considered in connection with bimetallic strips deals with conditions as often used in practice. heated, and allowed. to curve at the section under consideration, and in so doing exerts a force on its abutments corresponding to a moment at this section. In this case the maximum fibre stress is still on the center line, independent of curvature.

Next, consider the special strip shown in Fig. 2 in which the composition varies gradually from one surface to the other. Originally the stresses in this strip are the same as' in the bimetallic strip, provided neither is heated.

When this strip,-still unheated, is bent into an are, its stresses are exactly as those of the bimetal strip.

But if such a strip as shown in Fig. 2 be clamped flat and heated, the stress varies from maximums at the surfaces to zero at neutral axis and in this respect difl'ers from the bimetal strip under similar conditions.

Consider now this improved type of strip heated, but left free, so that it is not subjected to external forces. It will bend into an arc. Furthermore, there are no internal stresses whatever inasmuch as all incipient stresses mutually cancel one another. This is a very desirable situation. The high mid- The strip is' plaifie stresses have been entirely done away wit A free bimetal strip, if heated sufliciently, will take on a permanent set when the internal stresses, exceed theelastic limit of the material but with the type of strip here considered (Fig. 2) this damage to a free strip cannot occur.

If the strip of Fig. 2 be considered as heated and exerting a moment on its abutments, the internal stresses will be simply those caused by the external constraints.

The strip may be considered as being brought to its final condition in two steps. First it is allowedto curve freely when heated. This produces no internal stress. Second it is subjected to the external forces and bent to the final position. This produces the stress abovereferred to as caused by the external constituents. Thus if the strip is allowed to curve at all as it heats, the final maximum stress will be less than that obtained in the similar case for the bimetal. In other words, the strip can be injured by overstress only if the forces applied to it externally are too great for it to support when considered as a simple beam. These strips having the same modulus of elasticity,

and the same coefficients of expansion of the extreme fibres, and it is here assumed that the elastic limit in the graduated strip is as high as in the bimetal. It has also been assumed that the modulus of elasticity was the same throughout the material. It "is possible to make an ideal graduated material even when this is not the case. Consider any strip in which the expansion coefiicient varies linearly with distance through the strip. Upon heating, unclamped, each layer will expand an amount which varies linearly. The stripwill therefore deflect into a curve without internal stress. If external forces are now applied, these will produce internal stressesto correspond, and the amount of these and their distribution in the material will depend upon the values of the modulus of elasticity at different points in the strip. However, the principle still holds that the only stresses in the strip will be those produced by external constraints.

From the above, it will be seen that the strip of uniform gradation of expansion coefiicient may be approximated by a strip consisting of a number (preferably large) of discrete layers of slightly differing coeflicients arranged in the order of magnitude of these coefficients. When the number of layers is large the analysis will be, to sufli-- cient accuracy, that of the uniform strip, and the same conclusions will apply.

There is a great benefit even with a smaller number of layers such as three, the stresses being much less than with two metals.

For best design it may often be well to utilize layers of unequal thickness. The optimum design for a given service, and with given available materials, may be made by constructing stress, strain diagrams, and choosing that one which gives the smallest extreme stress.

In Figs. 3 and 4 is shown an application of the invention wherein the improved sheet (Figs. 1 or 2) is shown stamped and preformed as a bulged disc or cup C, after the manner of United States Patent 1,448,240, issued March 13, 1923. This disc will snap from the curvature shown to a reverse curvature upon change in temperature between predetermined limits, thus making and breaking contact between terminal engaging elements 13 (on the disc C) and line terminals 15 of a circuit 17. The disc 0 is supported at a central opening by a post 19. A switch of this nature, made with the material described will stand more overheating, inasmuch as the internal stresses due to heat ing are less. It will be understood in this connection that the analysis given above was for strips and that for snapping discs certain peripheral tensions and radial compressions are introduced, but'the internal efiects of these are less deleterious in this improved material than in bimetallic material. The

slight stresses due to adjusting the post 19- 21 indicates the body of the valve having openings 23, 25 and within which is mounted the disc C. In this case holes 27 are used to permit communication between the openings 23, 25. The valve is shown atnumeral 29, and as adjustable at 31. Change in temper ature causes snap-acting reversal of curvature of the disc 0, thus opening the valve 29. This is done with the advantages, above set out. Many other applications are presumed to come within the purview of this disclosure.

A point to be observed is that .by using more than just two sheets of material in a thermostatic sheet, more than a one step gradient of thermal coeiiicient of expansion is attained, that is, a plurality of steps are used. These cooperate with one another so as to reduce or eliminate internal stresses, such as was not possible to be done in the case of a bimetallic material.

It will be appreciated that the plating method is not the only one which may be used for effecting a gradual variation in composition. Liquid meta-l deposition may be used or rolling until exceeding thin leaves are infected or fusion or diffusion of metal or the ike.

In view of the above, it will be seen that the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limitingsense.

I claim:

1. A thermostatic sheet comprising joined layers of materials having progressive difierent thermal coefficients of expansion each pair of layers having substantial additive thermostatic activity in the combination, and the number of layers being greater than two.

2. A thermostatic sheet comprising joined discrete layers of materials having progressively different thermal coefiicients of expansion each pair of layers having substantial additive thermostatic activity in the combination, the number of layers being greater than two and the layers being arranged in order of magnitude of said coefficients.

3. A composite, thermostatic sheet comprising a number of joined layers of material adapted to provide a plural step gradient of progressively arranged thermal coefiicients of expansion all of which are active additively to effect increase in thermostatic action.

4. A thermostatic sheet comprising joined layers of material, the number of layers being greater than two, said layers having different thermal coefiicients of expansion varying progressively by equal amounts between successive layers.

5. A thermostatic sheet comprising materials of varying thermal coeflicients of expansion. the variation being continuous and gradual and in the direction of sheet thickness.

6. A thermostatic sheet comprising materials of progressively varying thermal coefficients of expansion, the variation being by a plurality of discrete steps, the combination of materials at each step being such that the thermostatic tendencies of all steps are all in the same direction.

7 A thermostatic sheet comprising materials of varying thermal coefiicients of expansion, the variation being by a plurality of progressive gradations and in the direction of sheet thickness, the material being progressively arranged according to said coefiicients whereby all tendencies to move upon temperature change are in. a predetermined direction.

8. A thermostatic sheet comprising materials of varying progressive thermal coeflicients of expansion, the variation being by a plurality of steps and in the direction of sheet thickness.

9. A thermostatic sheet comprising material gradually varying in its composition by infinitesimal degrees through the thickness of the sheet.

10. A thermostatic sheet comprising-material adually varying by infinitesimal degree in t ermal coefficient of expansion'through the thickness of the sheet.

11. The method of forming a thermostatic sheet comprising depositing by infinitesimal increments materials having a progressive change in thermal coefiic'ients of expansion.

12.. The method of forming a thermostatic sheet comprising depositing by electro-deposition materials havlng a progressive change in thermal coefiicients of expansion.

- 13. A composite thermostatic sheet comprising a plurality of discrete layers of materials of progressively different thermal coeflicients of expansion, the number being such as to approximate the effect of a gradual prono gressive change of thermal coeflicient of expansion across the thickness of the sheet;

In testimony whereof, I have signed my name to this specification this 7th day of August, 1930.

VANNEVAR BUSH. 

